In the curtain coating method for applying liquid compositions to a moving web, one or more distinct layers form a free-falling curtain and impinge on a moving web thereby coating the web. The distinct layer or layers may be formed by means of either a slide or extrusion hopper as described in U.S. Pat. Nos. 3,508,947 (Hughes) and 3,867,901 (Greiller). In order to prevent contraction of the falling curtain, it is necessary to provide edge guides at the longitudinal edges of the curtain to maintain the width of the curtain along its length. The edge guides can be positioned outside the width of the web to be coated so the entire width of the web is coated, or the edge guides can be located inboard of the edges of the web so as to leave an area of uncoated web at each longitudinal edge. This is known as the "internal" curtain coating edging process. The current state of the art of the internal edging process shown in FIG. 1, is described in U.S. Pat. No. 4,830,887 to Reiter assigned to Eastman Kodak Company.
In U.S. Pat. No. 4,830,887, a slide coating hopper 10 has two bent slotted tubes 50 as the edge guides. The tubes 50 are positioned so that the coating width is less than the width of the web or support 18. The free-falling composite curtain 12 extends transversely of the path of the moving support 18, drops over a height "h", and impinges onto the moving support 18 to form a multilayer coating. Support 18 is guided around a coating roller 8 where the curtain 12 impinges onto the support. A low viscosity flushing liquid 21, preferably water, is delivered at the top of the slotted edge guide 50 and distributed over the entire height of the edge guide from the coating edge 15 to the point where the slotted edge guides bend upwardly, just above the point where the curtain 12 impinges on the support 18. FIG. 2 is a cross-sectional view of the free-falling curtain 12, showing the slotted edge guide 50 with the flushing liquid 21. The width of the flushing solution 21 adjacent the free falling curtain is typically about 1-2 mm. At the bottom of the edge guide 50 a vacuum 53 removes substantially all of the flushing liquid 21 and even a small amount of the free falling curtain 12 before the curtain impinges on the moving web or support 18 as shown in FIG. 3.
Since the edge guides described above are stationary, the falling curtain will experience drag in the areas adjacent to the edge guides. The fluid velocity in the areas adjacent to the edge guides will be substantially reduced relative to the center portion of the curtain that is essentially in free fall. If the width of the flushing solution adjacent to the edge guides is too narrow, the area of reduced velocity will necessarily extend into the edge portions of the main body of the free-falling curtain that typically includes various photographic compositions. Further reductions in the velocity of the edge portions of the curtain can be caused by the suction system at the bottom of the edge guide. Any reduced velocity in the edge portions of the curtain causes these portions to impinge onto the moving web with less momentum relative to the center portion of the curtain. This causes the edge portions to be prone to air entrainment or otherwise coat in an unsteady, ragged, or wavy manner. In extreme cases, the reduction in velocity in the edge portions of the curtain can cause the curtain itself to be unstable at the bottom of the edge guides and may even break spontaneously from the edge guides. These problems limit the coating speed and minimum coating thickness and are exacerbated when the viscosities of the liquid compositions are high.
The patent of Ridley (U.S. Pat. No. 4,019,906) is an attempt to provide means to have a curtain that is stable at the edge guides when the central portion of the curtain has a low flow rate. Two edge curtains are formed using two additional coating hoppers 2, 3 as shown in FIG. 4. The curtain is stabilized along the edge guides 4 by maintaining a high flow rate per unit width in each of the edge curtains 9, 10, allowing the flow rate in the central portion of the curtain to remain low. This method has several serious disadvantages. First, it would be very difficult or even impossible to retrofit an existing curtain coating hopper with the additional coating hoppers 2, 3. Thus, it would be necessary to fabricate new hoppers to practice the method. The fabrication of new hoppers is an extremely expensive and time-consuming process. In addition, the method is not capable of performing internal edging. Moreover, the surface tension, viscosities and flow rates of the stripe must all be selected to preserve the stripe width on the hopper slide. Furthermore, no means are provided to apply a flushing solution to the edge guides. Thus, contamination problems can be expected when the edge curtains contain solutions that congeal or solidify such as aqueous gelatin solutions. The edge curtains may also break away from the guide, particularly if the viscosity of the auxiliary curtain is high. Finally, the edge curtains are rather wide (&gt;5% of the main body that contains the photographic compositions). This limits the yield of photographic product that can be produced on a given coating machine and results in increased costs due to the waste associated with the edge curtain composition.
The present invention solves the problems of the prior art outlined above and allows for curtain coating of very low flow rates per unit width that was not possible with the prior art method and apparatus. The method allows more latitude for choice of stripe flow rate and viscosity which can be of benefit in increasing speeds or in achieving lower flow rate curtains.